During the drilling of blast holes (for example at a depth of 15-30 m) often rotary drill bits are used, which work with pressurized air as flushing medium. A primary object for the flushing air is to lift the drill cuttings crushed by the drill bit, from the drilled hole. That is done by blowing air out through one or more nozzles terminating downwards in the drill bit the nozzles having a comparatively large diameter (for example 8-26 mm). Thereafter, the air is forced to rise up through the drilled hole at a fairly high speed. Another object of the flushing air is to cool the bearings for the hubs which are included in the drill bit, and to keep these bearings clean. To this end, each of the so-called legs of the drill bit on which the hubs are mounted, includes channels which terminate in areas of the bearings and through which a certain amount of the flushing air is conveyed from the main air flow through the drill bit. In order to obtain good drilling results it is vital that the air flow rate through the drill bit and the drilled hole be optimally adjusted for each individual case. This rate depends on many different variable factors and must therefore be controlled at repeated occasions during each individual drilling work. Some minerals can give coarse and heavy drill cuttings, which require a very high flow rate (for example about 50 m/s) to be lifted up from hole, while other minerals give fine-grained and light drill cuttings which require a considerably more moderate flow rate (for example about 25 m/s). The rate should on the other hand not be too high, since then abrasive damage to different components in the drill bit may occur. Usually the compressor capacity deteriorates with increasing life. For that reason it is not practically possible to rely on nominal ratings for the compressor when the real flow rate is assessed. It is consequently necessary to determine the flow rate in each individual case in accordance with the previously described prior art.